Environments of QSOs at Redshifts 0.9 to 1.3

📝 Abstract

We analyse new deep g and i-band imaging with the CFHT of 16 QSOs in the redshift range 0.9 to 1.3. The principal points of interest are the symmetry and signs of tidal effects in the QSO hosts and nearby (`companion’) galaxies. The sample measures are compared with similar measures on randomly selected field galaxy samples. Asymmetry measures are made for all objects to g ~22, and magnitudes of all galaxies 2 magnitudes fainter. The QSOs are found in denser environments than the field, and are somewhat offset from the centroid of their surrounding galaxies. The QSO hosts appear more disturbed than other galaxies. While the QSO companions and field galaxies have the same average asymmetry, the distribution of asymmetry values is different. QSO companions within 15 arcsec are fainter than average field galaxies. We discuss scenarios that are consistent with these and other measured quantities.

💡 Analysis

We analyse new deep g and i-band imaging with the CFHT of 16 QSOs in the redshift range 0.9 to 1.3. The principal points of interest are the symmetry and signs of tidal effects in the QSO hosts and nearby (`companion’) galaxies. The sample measures are compared with similar measures on randomly selected field galaxy samples. Asymmetry measures are made for all objects to g ~22, and magnitudes of all galaxies 2 magnitudes fainter. The QSOs are found in denser environments than the field, and are somewhat offset from the centroid of their surrounding galaxies. The QSO hosts appear more disturbed than other galaxies. While the QSO companions and field galaxies have the same average asymmetry, the distribution of asymmetry values is different. QSO companions within 15 arcsec are fainter than average field galaxies. We discuss scenarios that are consistent with these and other measured quantities.

📄 Content

arXiv:0901.0723v1 [astro-ph.GA] 6 Jan 2009 Environments of QSOs at redshift 0.9 to 1.31 J.B. Hutchings, P. Scholz Herzberg Institute of Astrophysics, 5071 West Saanich Rd., Victoria, B.C. V9E 2E7, Canada; john.hutchings@nrc.ca L.Bianchi Dept of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD21218 Received ; accepted 1Based on observations obtained with the Canada France Hawaii Telescope, which is operated by CNRS of France, NRC of Canada, and the University of Hawaii – 2 – ABSTRACT We analyse new deep g and i-band imaging with the CFHT of 16 QSOs in the redshift range 0.9 to 1.3. The principal points of interest are the symmetry and signs of tidal effects in the QSO hosts and nearby (‘companion’) galaxies. The sample measures are compared with similar measures on randomly selected field galaxy samples. Asymmetry measures are made for all objects to g ∼22, and magnitudes of all galaxies 2 magnitudes fainter. The QSOs are found in denser environments than the field, and are somewhat offset from the centroid of their surrounding galaxies. The QSO hosts appear more disturbed than other galaxies. While the QSO companions and field galaxies have the same average asymmetry, the distribution of asymmetry values is different. QSO companions within 15 arcsec are fainter than average field galaxies. We discuss scenarios that are consistent with these and other measured quantities. – 3 – 1. Introduction and observations Imaging studies of QSOs have ranged from the local universe to those at redshift near to 6. From these and related studies, it is widely accepted that nuclear activity arises from accretion episodes on to massive central black holes in galaxies, and there are many scenarios for the process. Given the evidence for the connections between galaxy stellar properties and the central black hole mass, there is interest in studying QSO episodes in galaxies, at all redshifts, for clues as to how the black hole and galaxies develop this connection (e.g. Salviander et al 2007). At low redshifts, there is considerable evidence that QSO host galaxies have had recent merging of tidal disturbance events, which activate the nuclear accretion process. At redshifts 2 and higher, host galaxies appear to be in their early stages of assembly, with high star-formation. Thus, these probably represent different stages in the evolution of both galaxy and central black hole. There are few environmental studies of QSOs at redshifts near 1. Falomo et al (2004) and Kotilainen et al (2007) discuss samples at redshift 1.2 and higher, while Kukula et al (2001) investigated a sample of 9, with redshift 0.8 to 1.0. There are several investigations at redshifts 2 and somewhat higher (e.g. Ridgway et al (2001), Hutchings et al (2002)). Most of these investigations were done at NIR wavelengths, where the contrast with the nucleus is expected to be better, and where ground-based AO works best. The main focus of these papers is the nature of the host galaxy, and there is some consensus that radio-loud objects have higher luminosity hosts, but many are luminous elliptical-type galaxies. Lacy (2006) gives a good summary of the work on higher redshift QSOs to that date. The redshift range around 1 is when large galaxies are assembled, and may be in the process of forming the disk structures common in the present epoch. This work describes an investigation of QSO host galaxies and environments to learn more about this stage of their evolution, based on a new and relatively bias-free sample. Our resolution and – 4 – wavelength range are not optimised for host galaxy details, and the main focus of this paper is galaxies that may be associated with the QSOs, and their morphology. In particular, we are interested in the asymmetry of such galaxies (and QSO hosts), which can be characterised by deviations from elliptical contours with radius, even if small scale details are not well resolved. We are particularly interested in asymmetry of the fainter outer parts of galaxies, which arise from recent merging and tidal events among them. Few QSO studies in this redshift range have discussed this aspect. Wold et al (2001) discuss the galaxy environments of QSOs at redshift 0.5 to 0.8 in a general way. Hutchings and Proulx (2008) discuss companion galaxy asymmetries in a low redshift sample, and this paper is a similar investigation near redshift 1. The sample is derived from the combined SDSS and GALEX surveys, which overlap in the shallow and medium GALEX surveys (see e.g. Bianchi et al 2007). Initial samples of some 3000 and 14000 objects, respectively, were drawn from two-colour planes such as FUV-NUV vs NUV-r, in the locations where QSOs separate out from stars and galaxies (see Bianchi et al 2005; Bianchi 2008). Within these is a subsample of about 700 objects identified as QSOs by their SDSS spectra. We selected those in the redshift range 0.9 to 1.3, and declination 10o to 40o, for imaging with CFHT. Observations were m

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